refactor(TeacherAPIController): Extract and refactor TeacherRegistrationAPIController

[Aaron-Detre]

Changes

• Moved createTeacherAccount method into new TeacherRegistrationAPIController class.
• Refactored createTeacherAccount and helper methods.
• Moved related tests into new TeacherRegistrationAPIControllerTest class.

Test

• Everything should work the same as before. My local environment doesn't have Google/Microsoft accounts or ReCaptcha, so I haven't been able to test those cases. I tried not to change any functionality, so I think it should all still work.
• A welcome email should still be sent when you create a new account.